Abstract

We study the semi-annual variation in geomagnetic activity, as detected in the geomagnetic indices am, aaH, AL, Dst and the four aσ indices derived for 6-hour MLT sectors (around noon, dawn, dusk and midnight). For each we compare the amplitude of the semi-annual variation, as a fraction of the overall mean, to that of the corresponding variation in power input to the magnetosphere, Pα, estimated from interplanetary observations. We demonstrate that the semi-annual variation is amplified in the geomagnetic data compared to that in Pα, by a factor that is different for each index. The largest amplification is for the Dst index (factor ~ 10) and the smallest is for the aσ index for the noon MLT sector (aσ-noon, factor ≈ 1.1). By sorting the data by the prevailing polarity of the Y-component (dawn-dusk) of the Interplanetary Magnetic Field (IMF) in the Geocentric Solar Equatorial (GSEQ) reference frame, we demonstrate that the Russell-McPherron (R-M) effect, in which a small southward IMF component in GSEQ is converted into geoeffective field by Earth’s dipole tilt, is a key factor for the semi-annual variations in both Pα and geomagnetic indices. However, the variability in the southward component in the IMF in the GSEQ frame causes more variability in power input to the magnetosphere Pα than does the R-M effect, by a factor of more than two. We show that for increasingly large geomagnetic disturbances, Pα delivered by events of large southward field in GSEQ (known to often be associated with coronal mass ejections) becomes the dominant driver and the R-M effect declines in importance and often acts to reduce geoeffectiveness for the most southward IMF in GSEQ: the semi-annual variation in large storms therefore suggests either preconditioning of the magnetosphere by average conditions or an additional effect at the equinoxes. We confirm that the very large R-M effect in the Dst index is because of a large effect at small and moderate activity levels and not in large storms. We discuss the implications of the observed “equinoctial” time-of-year (F) – Universal Time (UT) pattern of geomagnetic response, the waveform and phase of the semi-annual variations, the differences between the responses at the June and December solstices and the ratio of the amplitudes of the March and September equinox peaks. We also confirm that the UT variation in geomagnetic activity is a genuine global response. Later papers will analyse the origins and implications of the effects described.

Highlights

  • The semi-annual variation in geomagnetic activity has long been recognized (Broun, 1848; Sabine, 1852; Cortie, 1912; Chapman & Bartels, 1940; McIntosh, 1959; Russell & McPherron, 1973; Cliver et al, 2002; Le Mouël et al, 2004) and gives peaks at times close to both equinoxes in both the average levels of geomagnetic activity and the occurrence frequency of large geomagnetic disturbances

  • Other geomagnetic indices that do not have such a uniform F-Universal Time (UT) response, or which quantify effects that depend on integrated solar wind forcing, tend not to show the same F-UT activity pattern, it can be identified in some variations derived from them

  • We have shown that variability in southward field normal to the equator equatorial plane contributes over twice as much to the variability in the power input to the magnetosphere as does the Russell-McPherron (R-M) effect but, the semi-annual variation is almost entirely due to the R-M effect

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Summary

Geomagnetic data

We demonstrate that the semi-annual variation is amplified in the geomagnetic data compared to that in Pa, by a factor that is different for each index. By sorting the data by the prevailing polarity of the Y-component (dawn-dusk) of the Interplanetary Magnetic Field (IMF) in the Geocentric Solar Equatorial (GSEQ) reference frame, we demonstrate that the Russell-McPherron (R-M) effect, in which a small southward IMF component in GSEQ is converted into geoeffective field by Earth’s dipole tilt, is a key factor for the semi-annual variations in both Pa and geomagnetic indices. We show that for increasingly large geomagnetic disturbances, Pa delivered by events of large southward field in GSEQ (known to often be associated with coronal mass ejections) becomes the dominant driver and the R-M effect declines in importance and often acts to reduce geoeffectiveness for the most southward IMF in GSEQ: the semi-annual variation in large storms suggests either preconditioning of the magnetosphere by average conditions or an additional effect at the equinoxes. Later papers will analyse the origins and implications of the effects described

Introduction
The Russell-McPherron effect
The Equinoctial Effect and the role of solar wind dynamic pressure
The axial effect
Power input to the magnetosphere
Statistics of the am index as a function of Time-of-Year and UT
F-UT patterns in other Geomagnetic Indices
UT variation in the magnetosphere
Findings
Conclusions
Full Text
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